1. Field of the Invention
The present invention relates to a ball screw, and more particularly to a fixed torque preload piece that is located between two nuts of the ball screw, its design can reduce the rigidity of the preload piece and prevent deformation, and the pushing motion of the steel balls can generate an adjustable preload and can effectively absorb the sudden increase of pressure.
2. Description of the Prior Art
Nowadays, with the advancement of various precision feeding system, various linear motion mechanisms have widely come into human life, manufacturing factory and high science and technology instruments, such as linear guideway, ball screw, and the like. Although the technology for linear mechanism is developing fast, consumers still need the technology to be improved constantly. Therefore, there are still some problems to be solved. And this desire for constant improvement of technology is particular strong in many high precision mechanism-manufacturing fields that require comparative stability and quiet. Hence, the existing linear mechanism still has some technological blind spots to be improved.
In production of a ball screw of a linear product, the thread pitch will be changed because of the manufacturing error or the change in the raw material itself. Such change in thread pith will cause instability when the steel balls are rolling between the nut and the screw shaft, and the preload subjected on the linear product itself will become uneven, causing output fluctuation (decrease in precision of the mechanism) and overly abrasion (preload will go off gradually). Therefore, preload element starts to appear in the ball screw product.
In order to produce a preload between the nuts and to make the two nuts rotate synchronously on the screw shaft, a conventional preload structure has a preload piece arranged between two nuts, and each of the two nuts and the preload piece is milled with a keyslot, then a clamp pad is inserted in the keyslot and fixed by screws, so as to maintain a preload between the two nuts for enabling a synchronous rotation of the two nuts. The aforesaid structure adjusts the preload between the two nuts by the thickness of the preload piece. In other words, the conventional technology utilizes the keyslot and the clamp pad as a connection structure between the two nuts, thus preventing the nuts from rotating relative to each other, and the thickness of the preload piece is used to adjust the preload between the two nuts.
After long time of study and research, we found that the abovementioned conventional connection structure between the double nut and the screw shaft still has some shortcomings to be improved.
First, the thickness of the preload piece is difficult to control. The preload of the ball screw has to do with the thickness of the preload piece. To control the preload value, it must adjust the thickness of the preload piece repeatedly. The repeating grinding work is very troublesome, and if the preload piece is not replaced periodically, the preload will go off.
Second, the flimsy preload piece will be fixed between the two nuts by cooperating with the clamp pad, the rigidity of the preload piece can produce a preload for pushing the two nuts outward. However, this rigidity of this conventional preload is too great, it will be unable to absorb sudden increase in preload, on the contrary, it will cause deformation of the element.
To solve the abovementioned problems, a conventional preload structure of a ball screw is as shown in FIG. 1 (U.S. Pat. No. 5,501,118), wherein two nuts 10 are synchronously screwed on the same screw shaft 11. To produce a preload between the two nuts 10 and to enable the two nuts 10 to rotate synchronously, a preload leaf spring 12 is arranged between the two nuts 10, and a clamp plate 13 is screwed on the nuts for connecting the preload leaf spring 12 and the two nuts 10 together. The conventional preload leaf spring 12 automatically adjusts the preload and absorbs the deformation force between the two nuts, and it has the following problems:
First, the elasticity and the thickness of preload leaf spring 12 are difficult to control, to control the preload value and to solve the abrasion of the preload leaf spring 12, the preload leaf spring 12 must be replaced periodically or grinded constantly.
Second, the flimsy leaf spring 12 generates a preload for pushing the two nuts 10 outward, however, if the rigidity of the preload leaf spring is not great enough, or if the sudden increase of the preload is too large, the screw shaft 11 and the two nuts 10 will likely be damaged.
In view of the aforesaid reasons, another improved preload structure for a ball screw appears on the market, as shown in FIG. 2 (U.S. Pat. No. 5,467,661), wherein two nuts 14 are synchronously screwed on the same screw shaft 15, two opposite preload lead springs 16 are disposed between the two nuts 14, and an annular positioning assembly 17 is disposed on the outer periphery of the two nuts 14. These two opposite preload leaf springs automatically adjust the preload and absorb the deformation force between the two nuts, however they still have the following problems:
First, the elasticity and the thickness of the two preload leaf springs 16 are difficult to control, and also to control the preload value and to solve the abrasion of the preload leaf springs 16, the preload leaf springs 16 must be replaced periodically or grinded constantly.
Second, on the outer periphery of the two nuts 14 is arranged the annular positioning assembly 17, and the annular positioning assembly 17 has too many components and is complicated in structure, this will cause a substantial increase in assembly and maintenance cost.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.